It seems you guys are pretty focused on the tuning as the cause of failure. I'm not saying you're wrong, or that it can't be. But, from what I've seen and experienced, the pattern goes like this.
output shaft breaks
A brace will (should) help tremendously to stop the rotation of the diff.
Managing the tq output can also help to reduce the rotation of the diff.
For what it's worth, I have a TH400 in my car mated to the C6 diff. Multiple trans brake launches in the low 1.3 60' and foot brake launches into the 1.2s and it's holding together fine. Of course, I have a 2.48 1st gear. What does the 6L80 have?
Good luck with your repairs fellas.